1. Field of the Invention
The present invention relates to a dual-mode ring resonator, which is arranged on a substrate as a planar conductor ring, to which an input conductor and an output conductor are coupled.
2. Prior Art
Dual-mode ring resonators, in which two degenerate resonance modes are excited in a conductor ring, are known from the article "EXPERIMENTAL INVESTIGATION OF DUAL-MODE MICROSTRIP RING RESONATORS", 20th European Microwave Conference 1990, pp. 901 to 906, and from IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 44, No. 5, May 1996, pp. 723 to 729. These dual-mode ring resonators are, for example used as bandpass filters, which should have an attenuation that is as high as possible in their attenuation band and an attenuation that is as low as possible in their passband. It is often desirable to make the transition band between the passband and the attenuation band comparatively smaller. This means that a bandpass filter is to be provided which has passband sides that are as steep as possible. A filter characteristic with steep sides or flanks is provided with the help of the dual-mode ring resonator, because this type of resonator has attenuation or blocking poles in the vicinity of the passband edges. The known dual-mode resonators are usually operated with their fundamental frequency modes. This type of fundamental frequency mode operation occurs when the length of the conductor ring is approximately equal to a single wavelength at the desired fundamental frequency. The comparatively small length of the conductor required for the fundamental frequency operation results in a small radius of curvature of the conductor ring, which results in radiation of a comparatively larger amount of electromagnetic energy. Moreover the load caused by the coupling of the input conductor and the output conductor is very high. These properties reduce the filter quality very greatly. An additional reduction of filter quality occurs because inhomogeneities in the conductor ring are needed for coupling both modes, as can be seen from both published references mentioned hereinabove. Because of the comparatively large radiation from the ring resonator, the resonance frequency depends strongly on the spacing of the ring resonator from the cover of a housing, from which the microwave radiation is fed to the ring resonator. The resonance frequency is subjected to undesirable changes due to fluctuations of the distance of the housing cover from the ring resonator, which are caused by thermal expansion of the housing or by mechanical oscillations.